The present invention relates to a method and an apparatus for detecting a boundary surface between a blood plasma and a blood corpuscle suspension which have been separated by centrifuge. Such method and apparatus may be applied to an apparatus for delivering the plasma and corpuscle suspension separately.
In a conventional method, a difference of D.C. resistance between respective samples, for example, a serum and a clot was used to detect the boundary surface therebetween. FIG. 1 is a schematic view showing a known apparatus for detecting the boundary surface. As shown in FIG. 1, a sample contained in a test tube 1 includes a serum 2 and a clot 3 which are separated by centrifuge. In order to extract the serum 2, a pair of electrodes 4 provided in an electrically insulating tube 5 and a suction nozzle 6 are immersed into the sample. The tube 5 and nozzle 6 are secured to a holder 7 which moves up and down. While the electrodes 4 and the suction nozzle 6 are moved gradually downward, a resistance between electrodes 4 is measured in a continuous manner by applying a D.C. voltage across the electrodes 4. The boundary surface between the serum 2 and the clot 3 is detected when the resistance is changed abruptly. In this case, since the serum 2 is a liquid-like material having a low viscosity but the clot 3 is a gel-like material having a high viscosity, a resistance ratio of the serum 2 to the clot 3 becomes large about one to several tens, so that the boundary surface can be easily and precisely detected. However, if an anticoagulant is added to the blood sample, the resistance of a blood corpuscle suspension is low and thus, the resistance ratio of a blood plasma to a blood corpuscle suspension is about 1 to 1.1; therefore it is not possible to detect the boundary surface between the blood plasma and blood corpuscle suspension easily and correctly by the known method.